Dark matter annihilation or unresolved astrophysical sources? Anisotropy probe of the origin of the cosmic gamma-ray background

The origin of the cosmic gamma-ray background (CGB) is a longstanding mystery in high-energy astrophysics. Possible candidates include ordinary astrophysical objects such as unresolved blazars, as well as more exotic processes such as dark matter annihilation. While it would be difficult to distinguish them from the mean intensity data alone, one can use anisotropy data instead. We investigate the CGB anisotropy both from unresolved blazars and dark matter annihilation (including contributions from dark matter substructures), and we find that the angular power spectra from these sources are very different. We then focus on detectability of dark matter annihilation signals using the anisotropy data by treating the unresolved blazar component as a known background. We find that the dark matter signature should be detectable in the angular power spectrum of the CGB from two-year all-sky observations with the Gamma Ray Large Area Space Telescope (GLAST), as long as the dark matter annihilation contributes to a reasonable fraction, e.g., $\gtrsim 0.3$, of the CGB at around 10 GeV. We conclude that the anisotropy measurement of the CGB with GLAST should be a powerful tool for revealing the CGB origin, and potentially for the first detection of dark matter annihilation.

Figure 1

The CGB spectrum from dark matter annihilation (dashed) and blazars with the best fit LDDE GLF (dotted). Total intensity is shown by the solid curve, and the data points are from the EGRET data [2].

Figure 2

Left: Angular power spectrum $l(l+1)C_l/2\pi$ of CGB from dark matter annihilation with substructures (a) $\alpha =1$ and (b) 0.7. The dotted and dashed lines show the contribution from the 1-halo and 2-halo terms, respectively, and the solid line is the total. Right: The same as the left panels but with smooth dark matter distribution (i.e., no substructure; AK06 [41]), with the minimum mass of (c) ${10}^6{M}_{\odot }$ and (d) ${10}^{-6}{M}_{\odot }$.

Figure 3

Angular power spectrum of the CGB from unresolved blazars expected from the EGRET data. Contributions from Poisson term $C_l^P$ and the correlation term $C_l^C$ with $b_B=1$ ($b_B=b_Q(z)$) are shown by the dotted and dashed (dashed-dotted) curves, respectively. The total contribution is shown as the solid curve for $b_B=1$.

Figure 4

The same as Fig. 3 but for the CGB anisotropy expected from GLAST data.

Figure 5

Angular power spectrum of the CGB from dark matter annihilation ($f_D^2{C}_{l,D}$; dashed), blazars ($f_B^2{C}_{l,B}$; dotted), and cross correlation ($2f_B{f}_D{C}_{l,BD}$; dashed-dotted) that would be measured by GLAST at $E=10\mathrm{GeV}$, for various models of the blazar GLF and various fractions of dark matter contribution to the CGB, $f_D$ (Table ). The adopted dark matter mass is 100 GeV and the gamma-ray emission is assumed to be dominated by the substructure. The total signal $C_l^s=f_D^2{C}_{l,D}+2f_B{f}_D{C}_{l,BD}$ is shown as the solid curve, while the corresponding GLAST errors ($\delta C_l^s$; for two years) are indicated as boxes. The signal is to be detected if it is larger than the size of errors ($C_l^s>\delta C_l^s$). The subhalo distribution in a halo of mass $M$ is assumed to be $⟨N|M⟩\propto M$.

Figure 6

The same as Fig. 5 but for $⟨N|M⟩\propto M^{0.7}$.

Figure 7

The same as Fig. 5, but in the case of the host-halo-dominated annihilation (no substructures; AK06 [41]). The minimum mass of dark matter halos is assumed to be ${10}^6{M}_{\odot }$.

Figure 8

The same as Fig. 7 but with minimum halo mass of ${10}^{-6}{M}_{\odot }$.